The invention relates to optical disc recording systems and, in particular, to an electromagnetic carriage and actuator assembly for focussing, tracking, and random access in optical disc players and recorders.
Optical data storage systems that utilize a focused laser beam to record and instantaneously playback information are very attractive in the computer mass storage industry. Such optical data storage systems offer very high data rates with very high storage density and rapid random access to the data stored on the information medium, most commonly an optical disc. In these types of optical disc memory systems, reading and writing data is often accomplished using a single laser source functioning at two respective intensities. During either operation, light from the laser source passes through an objective lens which converges the light beam to a specific focal point on the optical disc. During data retrieval, the laser light is focused on the recording medium and is altered by the information of the data storage medium. This light is then reflected off the disc, back through the objective lens, to a photodetector. It is this reflected signal that transmits the recorded information. It is thus especially important that, when information is being written to or read from the memory, the objective lens, and the exiting focused beam, be precisely focused at the center of the correct track so that the information may be accurately written and retrieved.
In order to attain a precise reading of the information stored on the disc, it is necessary to be able to move the objective lens in both a focussing (i.e., perpendicular to the plane of the disc) or Z direction in order to focus the laser beam to a small point of light on a precise location of the disc to write or retrieve information, and in a tracking (i.e., radial) or Y direction to position the beam over the exact center of the desired information track on the disc. Focus and tracking corrections may be effected by moving the objective lens in either the direction of the optical axis of the lens for focussing, or in a direction perpendicular to the optical axis for tracking.
In these systems, the position of the objective lens in the focus and tracking directions is commonly adjusted by control systems. Actuators support the objective lens and convert position correction signals from the feedback control systems into movement of the objective lens. As will be appreciated, failure to focus the light on a small enough area of the medium will result in too large a portion of the disc being used to store a given amount of information, or in too broad of an area of the disc being read. Likewise, the failure to precisely control the tracking of the laser light will result in the information being stored in the wrong location, or in information from the wrong location being read.
In addition to translation along the Z axis to effect focusing, and translation along the Y axis to effect tracking, there are at least four additional motion modes for the actuator, each of which reduces the accuracy of the reading and writing operations and is thus undesirable during normal operation of the system. These undesirable modes of motion are rotation about the X axis (an axis orthogonal to both the X direction and the Z direction), or pitch; rotation about the Z axis, referred to as yaw; rotation about the Y axis, called roll; and linear motion along the X axis, or tangential translation. Motion in these directions is often caused by motor and reaction forces acting on the carriage and/or actuator. These modes typically produce undesired movement during tracking or focussing operations which subsequently affects the alignment of the objective lens relative to the optical disc.
The present invention overcomes the drawbacks of the prior art devices by providing an apparatus for optical reading or recording information on an optical information medium, wherein as a result of the relative position of the components of the apparatus and the magnitude and application points of the forces exerted to track and focus, the objective lens through which information is read from the information medium, prevents the objective lens from being affected by most resonances, motor forces and reaction forces. This is critical in that in optical recording, displacements of 0.02 micrometers can easily be picked up.
Specifically, one aspect of the invention is an apparatus for optically reading or recording information on an optical disk which is rotatable about an axis of rotation, wherein the apparatus includes a frame, a carriage, a carriage drive, an objective lens, an objective lens holder and a focus drive. The carriage drive (i.e., those portions of the carriage motor which move relative the frame) drives the carriage along a path radial to the axis of the rotation. The objective lens is mounted on the objective lens holder so that the optical axis of the objective lens is within 0.5 millimeters of intersecting the center of mass of the carriage mass. The holder is movable relative to the carriage so as to move the objective lens along its optical axis. This movement is controlled by a focus drive (i.e., those portions of the focus motor which move relative to the carriage) which drives the holder so as to move the objective lens along its optical axis. For purposes of this application including appended claims, the carriage and actuator assembly is considered to be broken down into two mass groupings. The first is the xe2x80x9cfine motor massxe2x80x9d (i.e., the mass of all components suspended for freedom of movement from the carriage). The second is the xe2x80x9ccarriage massxe2x80x9d (i.e., the mass of all components which move with the carriage excluding the fine motor mass and any incidental connecting wiring not wholly supported by the carriage).
Another aspect of the invention is an apparatus for optically reading or recording information on an optical disk which is rotatable about an axis of rotation, wherein the apparatus includes a fine tracking drive (i.e., those portions of the fine tracking motor which move relative to the carriage) for driving the objective lens holder so as to make fine adjustments in the relative position of the objective lens along a path radial to the axis of rotation of the optical disk. The fine motor mass has a center of mass within 0.5 millimeters of a line parallel to the optical axis containing the center of mass of the carriage mass.
Another aspect of the invention is an apparatus for optically reading or recording information on an optical disk which is rotatable about an axis of rotation, wherein the apparatus includes a frame, a carriage, a carriage drive, an objective lens, an objective lens holder, a focus drive and a fine tracking drive, wherein the carriage mass and the fine motor mass each have a center of mass within 0.5 millimeters of the optical axis. Preferably, the carriage mass and the fine motor mass have respective centers of gravity within 0.5 mm of one another.
Another aspect of the present invention is an optical axis for optically reading or recording information on an optical disk which is rotatable about an axis of rotation having a frame, a carriage, a carriage drive, an objective lens, an objective lens holder, a focus drive and a fine tracking drive, wherein the net center of force exerted by the fine tracking drive is less than 0.2 mm from the optical axis. Another aspect of the invention is an apparatus for optically reading or recording information on an optical disk having a frame, a carriage, a carriage drive, an objective lens, an objective lens holder, a focus drive and a fine tracking drive, wherein the center of force exerted by the coarse tracking drive is less than 0.2 mm from said optical axis.
Yet another aspect of the invention is an apparatus for optically reading or recording information on an optical disk having a frame, a carriage, a carriage drive, an objective lens, an objective lens holder, a focus drive and a fine tracking drive, wherein the carriage drive exerts a pair of forces on the carriage to drive the carriage along a radial path and the distance between the center of mass of the carriage and the point upon which one of the forces acts is within 0.2 millimeters of the distance between the center of mass of the carriage and the point upon which the other carriage drive force acts.